1. Field of the Invention
This invention pertains generally to a class of materials useful in microminiature thermionic converter (MTC) applications and methods for manufacturing those materials. More specifically, this disclosure describes heterogeneous mixed phase materials having tailored electron emission properties, and methods of manufacture and deposition of those materials in a manner consistent with fabricating thin film emitters and collectors for use in MTCs. The materials exhibit low work function, emit electrons at temperatures that are low when compared with those associated with electron emission in traditional thermionic converter applications, do not require activation to temperatures beyond operational levels of microminiature devices for energy conversion and recovery, and exhibit durability and chemical stability when exposed to the atmosphere. Additionally, the materials are suited to manufacture using integrated circuit and micro-electromechanical systems fabrication techniques, and they can be used in constructing electrode structures and coatings in a single process deposition of a multi-component film that contains necessary constituents.
2. Description of the Related Art
Thermionic conversion has been studied since the late nineteenth century, but practical devices were not demonstrated until the mid-twentieth century. Thomas Edison first studied thermionic emission in 1883 but Schicter did not propose its use for conversion of heat to electricity until 1915. Although analytical work on thermionic converters continued during the 1920""s, experimental converters were not reported until 1941. The Russians, Gurtovy and Kovalenko, published data that demonstrated the use of a cesium vapor diode to convert heat into electrical energy. Practical thermionic conversion was demonstrated in 1957 by Herqvist in which efficiencies of 5-10% were reached with power densities of 3-10 W/cm2.
Thermionic emission depends on emission of electrons from a hot surface. Valence electrons at room temperature within a metal are free to move within the atomic lattice, but very few can escape from the metal surface. The electrons are prevented from escaping by the electrostatic image force between the electron and the metal surface. The heat from the emitting surface gives the electrons sufficient energy to overcome the electrostatic image force. The energy required to leave the metal surface is referred to as the material work function, xc3x8. The rate at which electrons leave the metal surface is given by the Richardson-Dushman equation:
J=AT2exp(xe2x88x92exc3x8/kT),
where A is a universal constant, T is the emitter temperature, k is the Boltzmann constant, and xc3x8 is the emitter work function. Large emission current densities are achieved by choosing an emitter with low work function and operating that emitter at as high a temperature as possible, with the following limitations. Very high temperature operation may cause any material to evaporate rapidly and limit emitter lifetime. Low work function materials can have relatively high evaporation rates and must be operated at lower temperatures. Materials with low evaporation rates usually have high work functions.
Choosing the correct electrode material is a key component of designing functional thermionic converters. The materials here disclosed have potential application in a wide range of thermionic conversion technologies, however they are especially suited to MTCs. An example of a class of MTCs wherein the materials of this invention find application is disclosed in a separate patent application Ser. No. 09/257,335, now U.S. Pat. No. 6,294,858 filed on the same day as the present application. That separate patent application is incorporated by reference herein in its entirety.
In all thermionic converters, once the electrons are successfully emitted, their continued travel to the collector must be ensured. Electrons that are emitted from the emitter produce a space charge in the interelectrode gap (IEG). For large currents, the buildup of charge will act to repel further emission of electrons and limit the efficiency of the converter. Two options have been considered to limit space charge effects in the IEG: thermionic converters with small interelectrode gap spacing (the close-spaced vacuum converter) and thermionic converters filled with ionized gas.
Thermionic converters with gas in the IEG are designed to operate with ionized species of the gas. Cesium vapor is the gas most commonly used. Cesium has a dual role in thermionic converters: 1) space charge neutralization and 2) electrode work function modification. In the latter case, cesium atoms adsorb onto the emitter and collector surfaces. The adsorption of the atoms onto the electrode surfaces results in a decrease of the emitter and collector work functions, allowing greater electron emission from the hot emitter. Space charge neutralization occurs via two mechanisms: 1) surface ionization and 2) volumetric ionization. Surface ionization occurs when a cesium atom comes into contact with the emitter. Volumetric ionization occurs when an emitted electron inelastically collides with a Cs atom in the IEG. The work function and space charge reduction increase the converter power output. However, at the cesium pressures necessary to substantially affect the electrode work functions, an excessive amount of collisions (more than that needed for ionizations) occurs between the emitted electrons and cesium atoms, resulting in a loss of conversion efficiency. Therefore, the cesium vapor pressure must be controlled so that the work function reduction and space charge reduction effects outweigh the electron-cesium collision effect. An example of an operational thermionic converter is that found on the Russian TOPAZ-II space reactor. These converters operate at the emitter temperatures of 1700 K and collector temperatures of 600 K with cesium pressure in the IEG of just under one torr. Typical current densities achieved are  less than 4 amps/cm2 at output voltages of approximately 0.5 V. These converters operate at an efficiency of approximately 6%. The control of cesium pressure in the IEG is critical to operating these thermionic converters at their optimum efficiency.
A variety of thermionic converters are disclosed in the literature, including close-spaced converters. (See: Y. V. Nikolaev, et al., xe2x80x9cClose-Spaced Thermionic Converters for Power Systemsxe2x80x9d, Proceedings Thermionic Energy Conversion Specialists Conference (1993); G. O. Fitzpatrick, et al., xe2x80x9cDemonstration of Close-Spaced Thermionic Convertersxe2x80x9d, 28th Intersociety Energy Conversion Engineering Conference (1993); Kucherov, R. Ya., et al., xe2x80x9cClosed Space Thermionic Converter with Isothermic Electrodesxe2x80x9d, 29th Intersociety Energy Conversion Engineering Conference (1994); and G. O. Fitzpatrick, et al., xe2x80x9cClose-Spaced Thermionic Converters with Active Spacing Control and Heat-Pipe Isothermal Emittersxe2x80x9d, 31st Intersociety Energy Conversion Engineering Conference (1996).) Previously demonstrated thermionic converters, however, have not been able to achieve the current densities and conversion efficiencies predicted for the present invention. Others"" efforts in the field of close-space converters demonstrate that expense and difficulty arise as a result of separately manufacturing and assembling at close tolerances the converter components such as the emitter, collector and spacers. Additionally, the assembly process results in relatively large converters with spacing between the emitter and collector of up to several millimeters. A large gap spacing between the emitter and collector causes the energy conversion efficiency to drop dramatically, often necessitating Cs vapor systems even in converters otherwise designed to be xe2x80x9cclose-spaced.xe2x80x9d Such vapor systems are usually large and cumbersome, and precise control of Cs vapor pressures needed to maximize conversion efficiency (ensuring that space-charge reduction effects outweigh electron-Cs collision effect) is difficult.
Miniature thermionic converters without ionized positive vapor in the IEG offer the simplest solution to thermionic energy conversion. The small IEG size itself reduces the density of electrons in the gap (and their resulting current limiting space charge). As alluded to above, the close-spaced converter has historically been difficult to manufacture for large-scale operation due to the close tolerances (several microns or even submicron interelectrode gap size) needed for efficient operation. As demonstrated in the separate application (Attorney Docket No. SD-5987.1) referenced above, however, large scale production and operation of these close-spaced converters is now possible using IC fabrication techniques. Also, the development of low work function electrodes that use materials such as those of the present invention eliminates the need for gas adsorption to lower the electrode work functions. For the reasons described above and others, there remains an unmet need for low work function materials suited to MTC applications, traditional integrated circuit manufacturing techniques and fabrication of electrodes and coatings using a single process deposition of a multi-component film wherein emission properties can be tailored to satisfy given operational requirements. Favorable materials for such purposes should emit electrons at relatively low temperatures, should not require activation to temperatures beyond operational levels of microminiature devices for energy conversion and recovery, and should exhibit durability and chemical stability when exposed to the atmosphere.
Accordingly, an advantage of the invention is that is provides a thermionic converter electrode material formed by modulated deposition comprising mixed oxides including a thermally ionizable species, a surface complex stabilizing species and a metal.
Another advantage of the invention is that is provides a method of manufacturing thermionic converter electrodes and electrode coatings wherein the method comprises the steps of depositing adjacent to one another a plurality of individual layers each including a thermally ionizable species, a surface complex stabilizing species and a metal, and heating those layers so that the metal coalesces in a matrix of heterogeneous oxide.
Yet another advantage of the invention is that it provides electrodes including materials formed by modulated deposition and comprising mixed oxides including a thermally ionizable species, a surface complex stabilizing species and a metal.
Yet another advantage of the invention is that the heterogeneous mixed phase materials provided exhibit enhanced durability and stability in the atmosphere as compared to alkaline earth oxides alone, and the materials exhibit desirable emission characteristics without having to anneal or activate above normal operational temperatures.
These and other objects of the present invention are fulfilled by the claimed invention which employs modulated deposition of material in superimposed layers or in other tailored configurations resulting in localized discontinuous metal or oxide structures. The materials contain oxides and metal which are heated to coalescence yielding a composite structure comprising metal particles of engineered dimensions suspended in heterogeneous oxide. This structure when created using IC manufacturing processes such as rf sputtering forms a suitable coating for electrodes in thermionic conversion applications, and especially microminiature thermionic conversion applications. Various metal and mixed oxide formulations are contemplated by the invention where the mixed oxides include a thermally ionizable species such as barium and a surface complex stabilizing species such as scandium. Tungsten and the other transition metals are examples of metals suited for use with the invention. Materials manufactured using this technique exhibit durability and stability when exposed to normal atmospheric conditions.
Additional advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.